This invention relates to a process for manufacture of trimellitic acid from 1,2,4-trimethyl benzene, commonly known as pseudocumene, and more particularly relates to a method of recovering pure trimellitic acid (TMLA) from the reaction mass obtained by the liquid-phase oxidation of pseudocumene by air or oxygen.
The process of this invention provides a commercial process for the manufacture of TMLA through the catalytic liquid phase oxidation of commercially available pseudocumene with air in the presence of acetic acid as reaction solvent, separation and recovery of crystalline trimellitic acid from the oxidation reaction effluent, thermal dehydration of trimellitic acid to its anhydride and hydrolyzing the trimellitic anhydride (TMA) to TMLA with a solvent comprising water or a mixture thereof with one or more low molecular weight carboxylic acids.
This invention further provides a process for the manufacture of pure TMLA where instead of hydrolyzing the TMA to TMLA in water or a mixture of water and one or more low molecular weight carboxylic acids as described above, a TMA/dimethylformamide (DMF) adduct is hydrolyzed. The TMA/DMF adduct is prepared by dissolving TMA in DMF at elevated temperatures and cooling the solution to precipitate a reasonably stable DMF/TMA adduct. This adduct, which is believed to be comprised of one molecule of DMF and one molecule of TMA, is treated with a mixture of water and one or more low molecular weight carboxylic acids to free the TMA in the form of its hydrolyzed product, trimellitic acid.
The following are the general advantages of our novel process, including the ability to improve the TMLA product color as evidenced by TMLA .DELTA.E values. TMLA has advantages over trimellitic anhydride in that TMLA is not a respiratory sensitizer. Our process improves the TMLA product purity as evidenced by the decrease in the bromine levels and by the decrease in the Esterification Gas Chromatography (EGC) detected impurity level. If the TMLA purity is calculated by difference from the levels of impurities detected by EGC, then it is readily seen that in most cases the TMLA purity produced by our novel process is over 99.0%. The high purity of the TMLA produced by our process will play a significant role in opening up new markets for TMLA, in which a product of 99%+purity is desired. The impurities present in largest concentration in commercial TMLA are the phthalic acids: orthophthalic acid (OA), isophthalic acid (IA) and terephthalic acid (TA). The impurities interfere with the use of TMLA in polymerization and plasticizer manufacture and, consequently, methods to eliminate them from TMLA are desired. Our novel process has accomplished this. Previous research has shown that it is not possible to achieve the separation of IA and TA through fractionation, however, in our novel process a reduction in the level of diacid impurities has been obtained. The main portion of this reduction is by removal of OA and IA. Table IV illustrates some of the advantages of our process.
Qualitative observations indicate that as the percentage of low molecular weight carboxylic acid increases in the hydrolysis solvent the tendency of TMLA to supersaturate in the solvent is decreased. In fact, at a 3 to 1 solvent ratio (solvent to TMA) in 95:5 wt % acetic acid:water, the TMLA actually precipitates out as it hydrolyzes.
Pseudocumene is oxidized with air mainly to a mixture of dimethylbenzoic acids in the presence of catalysis provided only by cobalt and/or manganese oxidation catalysts under liquid phase conditions using acetic acid as the reaction solvent. By the use of oxygen as oxidant and a combination of cobalt as metal oxidation catalyst and alpha-methylenic ketones as side chain oxidation initiators or promoters, pseudocumene is oxidized mainly to a mixture of 2-methylterephthalic acid and 4-methyl isophthalic acid in the presence of acetic acid solvent and under liquid phase conditions at atmospheric pressure. Catalytic liquid phase oxidation of pseudocumene with air can be accomplished in the presence of acetic acid solvent and the catalysis provided by the combination of heavy metal oxidation catalyst and a source of bromine as disclosed and claimed in U.S. Pat. No. 2,833,816. This oxidation method using a combination of heavy metal oxidation catalyst and a source of bromine to provide catalysis results in the production of a 92 weight percent TMLA filter cake product in a two hour reaction at 198.degree. C. (about 390.degree. F. ). The theoretical yield of TMLA from pseudocumene is 175 weight percent. However, the oxidation method of U.S. Pat. No. 2,833,816 has been developed to produce total TMLA yields in the range of 152 to 161 weight percent or about 87% to about 92% of theory based on the pseudocumene hydrocarbon feed. By total yield of TMLA is meant all of the TMLA in the oxidation reaction effluent.
Even though the more highly developed catalytic liquid phase air oxidation of pseudocumene by the method of U.S. Pat. No. 2,833,816 produces total trimellitic acid yields of 152 to 161 weight percent based on commercially available pseudocumene, there are also coproduced trimesic acid, iso- and terephthalic acids, 4-methylorthophthalic acid, 2-methylterephthalic acid, 4-methylisophthalic acid and formyl phthalic acids in amounts as to present substantial problems in the recovery of high quality trimellitic acid, dehydration of trimellitic acid to its intramolecular anhydride and recovery of that anhydride.
Another problem in the manufacture of TMLA through the oxidation of pseudocumene to TMLA in the presence of acetic acid comes from the relatively high solubility of TMLA in acetic acid. This solubility goes from about 1.0 pound per 100 pounds glacial acetic acid at 80.degree. F. to 6.5 pounds per 100 pounds glacial acetic acid at 220.degree. F. The presence of water in the acetic acid increases the solubility of TMLA so that in aqueous acetic acid solvent having 82 to 85% acetic acid and 18 to 15% water by weight there are dissolved at 80.degree. and 220.degree. F. about 3.2 pounds and 16.5 pounds TMLA per 100 pounds solvent. Ordinarily aqueous acetic acid of 90 to 98% (10 to 2% water) by weight is used in the oxidation as solvent not only because acetic acid of higher strength is more expensive to recover, but also because the presence of 2 to 10% water by weight substantially eliminates oxidation induction. During oxidation of the methyl groups to carboxylic acid groups water is produced as a by-product and is generally retained through the removal of heat of reaction by condensing the acetic acid and water boil-up from the liquid phase in the oxidation zone and returning as condensate to the oxidation zone. The aqueous acetic acid solvent in the effluent removed from the oxidation zone can contain about 10 to 25% water (90 to 75% acetic acid) by weight when the 90 to 98% aqueous acetic acid solvent is used in the weight ratios of 5 to 2 parts per part of pseudocumene. Thus at usual crystallization temperatures of 60.degree. to 120.degree. F. a substantial amount of trimellitic acid remains in solution.
For example, in Example II of U.S. Pat. No. 3,161,658 there is described the cooling to 100.degree. F. of an oxidation reaction effluent containing for each 500 parts acetic acid solvent 200 parts TMLA and 50 parts of pseudocumene oxidation intermediates. There was recovered 135 parts crystalline TMLA per 500 parts of acetic acid solvent. Thus, of the originally produced 200 parts TMLA there was left in solution 65 parts or 32.5%. This appears to have been an oxidation of pseudocumene conducted in the presence of acetic acid solvent in the ratio of about 3.5 parts solvent per part of pseudocumene. Higher ratios of solvent to pseudocumene would have caused a greater proportion of the total TMLA to remain in solution at 100.degree. F. For example, at a 5 to 1 solvent ratio 45% of the trimellitic acid produced would have remained in solution at crystallization and filtration temperatures of 100.degree. F.
U.S. Patent 3,161,658 provides one technique for recovering the TMLA remaining dissolved in the aqueous acetic acid mother liquor. This is done by adding the mother liquor to a pool of molten TMA (370.degree.-375.degree. F. ) and flashing off water and acetic acid vapors and drawing off from the molten pool liquid in an amount equivalent to the weight of solids charged with the mother liquor. This liquid draw off is solidified, ground and dissolved in a dialkyl ketone or aromatic hydrocarbon (the ketone solution must be filtered to remove insolubles) and the solution is combined with anhydride from dehydrated 100.degree. F. filter cake. The aromatic hydrocarbon solution is filtered to remove an insoluble oily residue and the filtrate cooled to 75.degree. F. to precipitate trimellitic anhydride. This anhydride can be added to the anhydride from dehydration of 100.degree. F. first filter cake. By simple flashing at 6 mm Hg absolute there is recovered a trimellitic anhydride product of 95% anhydride content, 95% pure in yields of 85 to 90% based on the trimellitic acid produced by the oxidation. However, the ketone and aromatic hydrocarbon solvents are flammable and their foregoing uses, although advantageous, do present fire hazards.
U.S. Pat. No. 3,096,343 teaches a method for isolating trimellitic acid or trimellitic anhydride. In this method TMA or TMLA is first dissolved in hot DMF to form a solution. This solution is cooled to precipitate a TMA/DMF or TMLA/DMF adduct and the adduct is separated from the remaining solution. The isolated adduct is heated to drive off the DMF leaving the TMA or TMLA. While this patent teaches a method for isolating TMA or TMLA, the patent does not teach or specify the purity levels that can be achieved by the disclosed process. As will be discussed in more detail subsequently, it has been determined that the method disclosed in the U.S. Pat. No. 3,096,343 does not provide TMLA of sufficient purity for modern applications of TMLA and that the process of the instant invention is a superior process in that a more highly pure TMLA is provided.
The intramolecular anhydride of TMLA has become a commercial starting material for surface coatings having the desired properties of high thermal decomposition, high temperature insulating properties and good resistance to chemical attack and are substantially insoluble. These surface coatings are obtained from prepolymers prepared, for example, from TMLA intramolecular anhydrides and polyamines. Because of the trifunctionality of the intramolecular anhydride the final surface coating product is a polyamide-imide. The intramolecular anhydride of TMLA also has become a starting material for solid foams obtained by reacting an isothiocyanate among other reactants with the intramolecular anhydride. Air and heat drying paints and enamels with hydrocarbon or water solvent vehicles are also prepared from the intramolecular anhydride of TMLA. For most of these uses, TMLA intramolecular anhydride of an anhydride purity of 98 to 99% is required.
For many commercial applications mentioned above, color of the TMLA has become an important specification. Highly colored brown, tan, or even yellow products may no longer be acceptable. Triethylene Glycol (TEG) color is a typical standard measure of this performance quality of TMLA. In this method, a reaction of the TMLA with a 300% molar excess of triethylene glycol is carried out at 500.degree. F. (about 260.degree. C. ) to produce a solution whose color is matched with APHA color standards. Reaction time is sixty minutes. A typical commercial product must have a TEG color of 170 or less.
The Finished Ester Color (FEC) test is another method for evaluating the color of TMA or TMLA. This test is similar to the TEG test described above, however, instead of preparing an ester of TMA (or TMLA) using triethylene glycol, an ester is prepared using 2-ethylhexanol. The color of the resulting ester is likewise evaluated using the APHA color standards.
A further method for evaluating the color of TMA or TMLA is termed the .DELTA.E method. This is a spectrophotometric method wherein the total color difference between a solution of 3N NaOH and a solution composed of 5 gm of TMA or TMLA dissolved in 30 ml of 3N NaOH is obtained. The .DELTA.E value is related to the color of the TMA product in the 400 to 700 nm wavelength range as measured by a spectrophotometer.
The problems that require solving are the recovery of TMLA in yields above 87 to 92 mole percent utilizing catalytic liquid phase oxidation of pseudocumene with air in the presence of acetic acid solvent, the increase of recovery of TMLA from the oxidation reaction effluent, an improved distillative and/or evaporative process for separating the intramolecular anhydride from the crude anhydride melt obtained by the dehydration of impure TMLA, elimination of the fire hazards accompanying the use of dialkyl ketones or aromatic hydrocarbon extract solvents previously disclosed for advantageous use in increasing the recovery of TMLA and the other problems before mentioned. The advantage of our novel process is to hydrolyze the TMA, or the TMA/DMF adduct, with aqueous acetic acid or water and recover pure TMLA which does not have the sensitization effects associated with TMA.
U.S. Pat. No. 4,587,350, incorporated by reference herein, discloses a process for the oxidation of pseudocumene to TMLA by a catalytic oxidation of pseudocumene with air in the presence of acetic acid in an oxidation zone in the liquid phase with catalysts comprising zirconium, cobalt, and manganese and a source of bromine.
The process of this invention provides an integrated system for the commercial production of TMLA.
A process for the manufacture of TMLA by the steps of catalytic oxidation of pseudocumene in the presence of acetic acid in an oxidation zone wherein liquid-phase conditions are maintained and the catalyst comprises one or more heavy metal oxidation catalysts comprising zirconium, cobalt, and manganese and a source of bromine, cooling the oxidation reaction effluent to crystallize TMLA, separating and recovering crystallized TMLA from the acetic acid solvent mother liquor, distilling from the acetic acid mother liquor to obtain a mixture of acetic acid and water for concentration of the acetic acid content to provide acetic acid solvent concentrate for recycle to the oxidation and to obtain a bottoms fraction having high melting solids, heating the crystalline TMLA to convert it to its anhydride and distilling the anhydride to obtain distilled TMA. The improvement arises from hydrolyzing in a mixture of acetic acid or other low molecular weight carboxylic acids and water the TMA to TMLA continuously or in a batch process. The ratio of acetic acid or other low molecular weight carboxylic acid to water is about 0 to about 19; preferably about 1 to about 19 by weight, and most preferably 19. The amount of carboxylic acid and/or water solvent used to hydrolyze the TMA to TMLA is not critical. However a weight ratio of from about 1 to about 10 is useful and a ratio of from about 3 to about 7 is preferred.
Alternatively, it is a TMA/DMF adduct that is treated with a mixture of acetic acid or other low molecular weight carboxylic acid, and water to form the purified TMLA. When purifying TMA this process comprises dissolving trimellitic anhydride (TMA) in hot dimethylformamide (DMF) to form a DMF solution, cooling the DMF solution to precipitate a TMA/DMF adduct, separating the TMA/DMF adduct from the remaining DMF, treating the TMA/DMF adduct with a mixture of acetic acid or other low molecular weight carboxylic acid and water to decompose the TMA/DMF adduct and hydrolyze the TMA to trimellitic acid, and separating the trimellitic acid from the remaining liquid to recover pure trimellitic acid.
The TMA/DMF solution may be first treated with activated carbon and filtered to remove the carbon and any insoluble impurities. In the mixture of the low molecular weight carboxylic acid and water used to decompose the TMA/DMF adduct the ratio of acid to water is about 1 to about 19, preferably about 3 to about 19 by weight. The amount of DMF that is required to dissolve the TMA is not critical. The amount used, however, must be sufficient to dissolve all or most of the TMA at a temperature of from room temperature to the boiling temperature of DMF at atmospheric pressure or if superatmospheric pressures are employed then at the boiling temperature of the DMF at the superatmospheric pressure. There should also be at least enough DMF to provide at least about 1 mole of DMF per mole of the TMA. Preferably the weight ratio of DMF to TMA should be from about 0.3 to about 6 and, preferably, the mixture of TMA and DMF is heated to from about 50 to about 160.degree. C. to dissolve or substantially dissolve the TMA.
The amount of the mixture of low molecular weight carboxylic acid and water required to decompose the TMA/DMF adduct, or the temperature or time required to effect this decomposition are not critical although the preferred method for decomposing the TMA/DMF adduct comprises heating the TMA/DMF adduct with a mixture of the carboxylic acid and water wherein the weight ratio of the mixture of carboxylic acid and water to the TMA/DMF adduct is from about 1 to about 7 and preferably from about 3 to about 5. The temperature for the decomposition preferably is in the range of from about 70.degree. to about 115.degree. and heating is continued for a time sufficient to complete or substantially complete the decomposition of the TMA/DMF adduct and hydrolyze the TMA to TMLA.
The low molecular weight carboxylic acids useful for this invention are those having from 1 to 4 carbon atoms and may be straight chain, branched, saturated or unsaturated. Examples of these carboxylic acids are formic, acetic, propionic, butyric, acrylic, crotonic, isocrotonic, vinylacetic, methyacrylic and isobutyric acid. Due mainly to cost and availability the preferred acids are formic, acetic, and propionic acid. Acetic acid is the most preferred. Mixtures of these acids having 1-4 carbon atoms are also useful.